Molecular Orientations in Mica and its Implication on Potassium Fertility of Soil

In sixties the work of Radoslovich and Norrish established distortions from the previously accepted ideal structures of mica. The main distortions was that the surface oxygen triads (the oxygens forming the bases of the tetrahedra) are rotated about c alternatively clockwise and anti-clockwise through an angle α from their positions in the ideal hexagonal arrangement. The cavities in the surface oxygen array are deformed from the hexagonal arrangement of the ideal model to a ditrigonal array, so the interlayer cations in micas have six nearest neighbours. Formerly the interlayer cations were thought to be in twelve-fold coordination, situated in the hole formed between two vertically opposed hexagonal rings of oxygens. Later on this theory was slightly modified showing that interlayer K—O distance was a variable in mica structures and was not the constant 2.81Å as assumed earlier. These along with the interlayer K-H(of octahedral OH) distance due to different orientations of octahedral OH group vary with mica composition and hence leads to different K release threshold levels. This in turn has implications on observation of two threshold levels, viz., Release threshold level and fixation threshold levels in soil which vary from soil to soil depending on the ratio of trioctahedral and dioctaheral components of mica. The range of labile K between these two threshold levels was termed Plateau of Labile K. If soil available K remain within in this range neither release nor fixation takes place. The difference between these two levels was found to be due to hysteresis effect between adsorption and desorption. High contribution of non exchangeable K to plant uptake, even in the presence of high available K was due to the fact that in the rhizosphere the available K went below the threshold level of K release. Simulation model showed that the response to applied potassium was found to be dependent on the release threshold levels of potassium.